Decarburization process for highchromium steel



United States Patent 3,ii46,lil7 DECARBUREZATEGN PROCEEES FQR li H'Gll-l- CHRQMiUM STEEL Edward 6. Nelson, Kenmore, and Neal R. Grilling, Grand island, N.Y., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Filed Nov. 18, 196i No. 84,956 Claims. (Ci. 75-59) The present invention relates to a decarburization process wherein a predetermined carbon content can be achieved without substantial oxidation of chromium and iron.

The present practice in high-chromium steel melting comprises several phases including (1) meltdown, (2) decarburization, (3) reduction of alloying elements from the slag formed during decarburization and (4) finishing or adjusting the final steel to specifications.

Pure oxygen blowing is commonly employed to decarburize chromium-containing steels. During decarburization a substantial portion of chromium as well as iron is oxidized into the slag in an eiiort to remove carbon to a low level without necessitating maintenance of temperatures in excess of the refractory limit.

A standard relationship is utilized today in chromiumcontaining steel making to correlate the required final temperature to be maintained during decarburization for a desired carbon content and to estimate the amount of chromium which it is possible to retain as metal in the steel melt at the desired carbon content and the tempera ture dictated by the particular refractory utilized. This relationship is shown in several publications; namely, Atomic interaction in Molten Alloy Steels, by Chipman, in Journal of Iron and Steel Institute, vol. 180 (1955), pp. 97-106, and Gbservations of Stainless Steel Melting Practice, by Hilty, Rossbach and Crafts, in Journal of iron and Steel Institute, vol. 180 (1955), pp. 116-128.

The relationship put forth in these publications is now commonly utilized in industry to guide chromium-containing steel production processes. This chromium-carbon-temperature relationship generally indicates that an increasingly high temperature is required to maintain chromium, without oxidation, in the steel as carbon is removed during decarburization. This relationship c ictates that refractory lining material be subjected to severe punishment by high temperatures to make low carbon-high-chromiutn stainless steels.

To partially circumvent refractory problems in an eifort to extend the number of heats the furnace lining will endure, the steel maker utilizes a lower temperature than required to prevent severe metallic oxidation and decarburizes the molten steel to the desired carbon content and accepts the oxidation of chromium and iron to the slag as an attendant evil. After decarburization, reducing agents such as ferro-alloys containing silicon are added to the melt to reduce the chromium and iron oxides from the slag and return the same to the steel melt. Additional carbon may be introduced into the melt from carbon electrodes during electric furnace operations; therefore, allowance must be made for this condition during decarburization. It is readily seen that acquiring the final carbon content at a desired chromium level is not as-well controlled a procedure as could be desired even in well-formulated steel making, and that conventional practice does not permit decarburization to low levels without substantial oxidation of chromium and iron.

It is an object of the present invention to provide a process for removal of carbon from molten steels containing 3 to 38 percent chromium without substantial oxi dation of chromium and iron.

dfi lhddl Patented July 24%, 1962 it is another object of the present process to provide for programmed decarburization of molten steels having from 3-30 percent chromium present as an alloying agent with a minimum oxidation of chromium and iron.

It is still another object of the preesnt process to provide programmed decarburization to a predetermined carbon content in molten steels where predetermined chromium contents ranging from 3 to percent are desired.

It is another object of the present process to decarburize molten chromium-containing steels at lower temperatures with less chromium and iron oxidation for a given desired final carbon and chromium content than has previously been possible.

The process accomplishing the above-mentioned objects comprises introducing gaseous oxygen and at least one inert gas selected from the group consisting of argon, xenon, helium, krypton and neon into a molten stainless steel inelt containing from 3 to 30 percent chromium to cause decarburization of the melt by reaction between oxygen and carbon and removal of carbon oxide. The ratio or" oxygen to inert gas is decreased during and in relation to decarburization in a controlled programmed manner whereby chromium and iron oxidation are cssentially minimized at temperatures utilized in highchromium steel production.

The maximum permissible oxygen concentration in the gas introduced into the melt is governed by the following variables; namely, chromium content, carbon content and temperature. it has been found that maintenance of a critical oxygen volume percent of the total selected inert gas-oxygen volume introduced into the melt will allow decarburization at various temperatures with minimum oxidation of chromium and iron from the steel. In addition, the oxygen volume percent in the oxygen-inert gas mixture can be critically adjusted to a value determined by the final desired and predetermined carbon and chromium content for given temperatures at the finishing point.

During the decarburization of chromiurncontaining steel, several program alternatives can be utilized to remove carbon to a predetermined level. They include (1) stepwise reduction of the oxygen volume percent in the gas mixture for decarburization to successively lower levels of carbon in given increments with the final predetermined carbon content being attained in the final program step; (2) one adjustment of the oxygen volume percent in the gas mixture to the volume percent required to achieve the desired predetermined carbon content, (3) continuous adjustment of the ratio of oxygen volume percent to inert gas volume percent as carbon is removed from the stainless steel melt, and (4) partial decarburi- Zation to a predetermined level with inert gas-oxygen mixtures by any one of the above alternatives, followed by a final decarburization to a desired carbon content by means of inert gas injection.

Alternative (3) will be most eflicient and advantageous from the standpoint of decarburization time but continuous adjustment of the ratio of oxygen volume percent to inert gas volume percent as carbon continuously decreases will be more diilicult. The first alternative would appear to be easier to maintain in production and the time for the total decarburization process would not be substantially greater than alternative (3). Alternative (2) will allow decarburization to predetermined levels at predetermined chromium contents but will require the relatively longer time for decarburization. Alternative (4) in which final decarburization is achieved by the iniection of inert gas alone makes it possible to obtain exceptionally low carbon levels of 0.01 percent and lower providing that preliminary decarburization with oxygenargon mixtures has been carried far enough. Another the melt at a given instant of time.

advantage of final decarburization with inert gas is that in any of the programs it will tend to remove finely divided metal oxides suspended in the melt and will also tend to remove hydrogen, the presence of which in excess amounts causes porous ingots.

The critical oxygen volume percent is dictated by the following relationship which applies for steel containing 3 to 30 percent chromium. It should be noted that a portion of this relationship is shown in the Hilty et al. article on p. 119. This portion of the relationship is utilized in conjunction with modifications which give the following relationship to indicate the maximum critical oxygen volume percent to be maintained during each of the several alternatives in programmed decarburization of stainless steels.

The relationship is as follows:

100Z Percent Cr 13,800 A C antilog 8.45 1

Oxygen volume percent is noted by percent This is the maximum volume percent of oxygen in relation to total volume of oxygen plus inert gas introduced into The weight percentages of chromium and carbon desired at the end of a given decarburization step are denoted by percent Cr and percent C, while the temperature in degrees Kelvin at the end of the step is represented by T.

The term Z represents a parameter, not recognized by the prior art, which has a value in the range of about 1.1 to 20.0. The recognition and evaluation of the parameter Z comprise further improvements over prior art because knowledge of'its value permits the calculation of the required volume percent oxygen in the inert gasoxygen mixture in order to obtain predetermined final carbon and final chromium contents at melt temperatures of practical interest. The value for Z may vary with varying decarburization practice such as dififering blowing rates and injection devices; however, we have found that this value is usually in the range between about 2 and for inert gas-oxygen injection as disclosed herein. Moreover, we have found that Z is related to the oxygen concentration in the gas mixture, for a given temperature and melt composition, by the expression (Z) (percent C )=130.0. This relationship substituted in the overall relationship then yields:

Percent 0 While the process of this invention prevents the severe metallic oxidation encountered with prior art oxygen lance practice, the metallic oxidation cannot be completely suppressed. A portion of the oxygen in the decarburizing gas mixture reacts with carbon and a portion with chromium and iron. The value for Z is a function of the relative oxidation of carbon and molten metal. In this respect, the higher the value for Z the more metal is being oxidized.

In each alternative the percent Cr in the above relationship is desirably maintained at a high level from the beginning of decarburization up to the achievement of the final carbon level, thereby minimizing chromium oxidation. In the stepwise decarburization to successively lower carbon contents down to the desired and predetermined carbon content by removing carbon in increments,

, the oxygen is maintained at a value equal to or less than the value dictated by the above relationship where percent Cr is the desired predetermined chromium content in weight percent at the end of each step, T is the temperature of the melt in K. at the end of each step and percent C is the lower percent. carbon by weight to be obtained during each stepwise reduction in carbon but at all times being not less than the final predetermined and desired carbon content in the finished product.

When the single adjustment method is utilized to decarburize, the variables dictating the volume percent oxygen are as follows: the percent Cr is the final predetermined chromium content desired in the decarburized steel; T is the final temperature in degrees Kelvin during decarburization; and percent C is the final desired carbon content in the decarburized steel. During this method of decarburization the volume percent oxygen introduced is maintained constant and equal to the above dictated value.

In the continuous adjustment method noted above as alternative (3), the volume percent oxygen is maintained essentially equal to the value dictated by the above relationship where percent Cr is the desired and predetermined chromium content after decarburization, T is the temperature during decarburization, and percent C is a constantly decreasing value as decarburization proceeds. In this method to maintain chromium content essentially constant at a substantially invariant temperature during decarburization, the required volume percent oxygen will be constantly decreasing in relation to the decreasing carbon content of the molten stainless steel melt.

In alternative (4) the inert gas injection step is used at the conclusion of the injection programs as disclosed by the other alternatives, said programs being concluded in this case preferably at a carbon level within about 0.04 percent or less of the desired final carbon content. The volume of inert gas required per ton of metal in this final step of alternative (4) depends on the absolute carbon level and also on the amount of carbon removed in this particular step. Since usually no more than about 0.08 percent oxygen exists dissolved in the melt after an oxygen-inert gas blow, stoichiometrically about 0.06 percent carbon can be eliminated without refractory damage by a subsequent inert gas blow. In actual practice the percent carbon eliminated may be somewhat less.

In all of the alternatives above the oxygen volume per-- cent may not be substantially greater than the value resulting from the relationship under the stated conditions or substantial losses in chromium will result due to oxidation of the same and passage of chromium oxide to the slag phase.

From the above relationship it is readily apparent that drastic or rapid cooling of the melt will result in a partial loss of the beneficial effects of the addition of inert gas and will lead to high chromium losses. The preferred method entails maintaining the temperature substantially constant or allowing it to increase during decarburization within a range of from 1700 to 2300 degrees Kelvin. Variance within the stated range can be tolenated Without substantial oxidation of chromium if the volume percent of oxygen being introduced into the melt is adjusted to compensate for the change in temperature. Again, the final temperature must be taken into account in the above relationship in arriving at the maximum permissible oxygen volume percent which is not to be exceeded for minimized chromium loss. For example, if a 20 percent chromium stainless steel is treated at about 1700 C. with a maximum volume percent oxygen of about 40 percent to decarburize to a level of about 0.07 percent carbon and during decarburization the melt is allowed to cool to 1600 C., the maintenance of 40 volume percent of oxygen introduction during blowing to 0.07 percent carbon at 1600 C. will result in a final chromium content of about 8 percent; however, if the melt temperature is maintained at 1700 C. a final Cr content of about 19 percent will be easily achieved. The difference in chromium content will be lost to the slag due to oxidation.

Slight decreases in temperature during the decarburization process may be anticipated and compensation can be made accordingly by programming the volume percent oxygen to correspond to the decreased temperature or by maintaining the oxygen volume percent low enough below the maximum volume percent for the higher temperature so as not to expose the melt to a ratio of oxygen to inert gas which will cause substantial oxidation of chromium for certain anticipated temperature decreases during decarburization of the steel. Generally, a temperature de crease will be undesirable and can be prevented by use of suificiently high blowing rates.

The time required for decarburization is effected by several variables including temperature and the rate of introduction of gases. Generally higher temperatures and higher blowing rates are conducive to shorter decarburization periods.

Moreover, we have found that the proper introduction of the gas mixture into the steel melt is essential for rapid and thorough decarburization reactions. The gas should be injected in the form of small single bubbles or a dispersion of small bubbles at least several inches below the melt level. In the preferred embodiment of this invention, the bubbles should not exceed 3 to 5 millimeters in diameter. This gives minimum gas consumption. It gas consumption is not of major importance, larger bubble sizes can be employed. However, small bubbles provide an extremely large metal surface area per unit amount of gas introduced into the melt. For example, one standard cubic foot of inert gas will be exposed to about 4000 square feet of molten metal surface if the bubble size is about 3 mm. The efiective surface area can be chosen essentially at will or as needed by a particular metal treatment by choosing the proper bubble size. The smaller the average bubble size, the larger the eilcctive metal-gas interface area for the process or" this invention. Since the mass transfer rate is dependent on the relative partial pressures of the components'to be transferred and also on the equilibrium partial pressures of a given component for the dissolved and gaseous state, the gas mixture, as employed in the practice of this invention, has an initial partial pressure of carbon monoxide equal to about zero, and in this manner, provides a large driving force for the rapid removal of carbon in the form of carbon monoxide. During normal practice of this invention, the residence time of a bubble in the melt is of the order of about 1 second per foot of melt depth; hence, the inert gas bubbles essentially get saturated with carbon monoxide during the bubbling process and are rapidly removed from the melt. However, every bubble gets the benefit of being introduced at essentially zero carbon monoxide partial pressure within the bubble.

The proper use of inert gases for the most effective results in the practice of this invention is not suggested by their apparent function in reducing the partial pressure of carbon monoxide in a gas mixture. In fact, any gas, inert or reactive, will fulfill this function; hence, for the purpose of this invention, an inert gas is not defined simply by the partial pressure laws. As used herein, the term inert gas is defined to mean a gas selected from the group consisting of helium, neon, argon, krypton, and xenon.

In this connection, it must be pointed out that prior art work based on results obtained with the use of nitrogen in pyrometallurgical processes also claims similar results with argon and helium. 'lhis is possibly due to the fact that nitrogen dissolves relatively slowly in molten iron under certain conditions so as to give an impression of chemical inertness to the non critical observer, which in turn, results in the classification of nitrogen as an inert We have found that the rate at which nitrogen is dissolved by molten iron and its alloys is influenced marked ly by the oxygen content of the metal. The lower the oxygen content, the more rapidly nitrogen will be ad sorbed. When carbon is being removed from molten steel by bubbling with nitrogen gas, the oxygen content, of course, is also reduced and nitrogen will be adsorbed at the same time. An important point that has not been recognized by the prior art is that at oxygen levels of about 0.03% and less in the stainless steel bath the rate of nitrogen pick-up will be rapid enough to raise signifi cantly the nitrogen content of the steel during processing.

Nitrogen in the steel is usually undesirable except in special cases, since it promotes blow holes, metal rising, and the like.

P or injection of the gas mixture according to the process of this invention, any suitable means such as ceramic tubes, conduits, nozzles, tuyeres, and the like can be employed. A relatively non-consumable material of construction is desired, since in this manner, the introduction of undesirable elements into the melt can be avoided. For effective operation of the decarburization process, the injetion device should cause the gas mixture to bubble thron the steel bath rather than to pass over its surface.

Injection devices with internal diameters of up to 0.5 inch may be employed.

Several specific embodiments of the present invention are put forth below.

A stepwise decarburization process may be conducted on molten stainless steel to remove carbon to about 0.04 percent at a final melt temperature of about 1700 C. when the stainless steel bath initially contains about 15 percent chromium and about 0.15 percent carbon. The carbon is reduced to 0.08 percent in the initial step utilizing about 50 volume percent oxygen and then the volume percent of oxygen is adjusted to about 35 percent to obtain the final desired carbon content of 0.04 percent in the final decarburization step.

Another illustration of stepwise decarburization from about 0.50 percent carbon to 0.04 percent carbon in molten stainless steel containing about 20 percent chromium at a temperature of decarburization of about l600 (I. may be conducted in three steps as follows: (1) reduce the carbon content from 0.50 percent carbon to 0.26 percent carbon by introducing an oxygen-inert gas mixture containing about 50 volume percent oxygen into the melt; (Z) adjust the volume percent of oxygen to about 30 volume percent and reduce the carbon content to about 0.11 percent; then (3) reduce the carbon content to the final desired carbon content of 0.04 by introducing a volume percent of oxygen of about 18 percent. Substantially no chromium will be lost in the above process.

A gas mixture having an oxygen concentration equal to the maximum permissible oxygen concentration at the final carbon content for a given melt temperature and chromium content may be injected throughout the decarburization. For example, at a melt temperature of 1800 C., a chromium content of 20 percent, and a desired final carbon content of 0.02 percent, a gas mixture containing about 30 percent oxygen is injected. No programming of the oxygen content is necessary in this manner. However, the gas injection period is usually longer as compared to that for programmed injection at approximately the same temperature.

From the above embodiments, it is readily realized that the present process permits reduction of carbon to a lower level than is shown in the prior art while retaining higher chromium contents at lower temperatures, in addition to enabling an artisan to adjust carbon contents in molten stainless steel melts to predetermined levels for specific chromium contents within the range of about 3 to 30 percent. By way of illustration, the above embodiment shows achieving a carbon content of 0.04 at a temperature of 1600 C. with a final chromium content of about 20 percent. The prior art processes would require a temperature of at least 2000 centigrade to produce the same product with conventional practice. if the prior art processes attempted to produce a 0.04 percent carbon stainless steel at a final temperature of 1600 C. starting with a chromium content of about 20 percent, nearly all the chromium would be oxidized, that is, only about 1 percent chromium would be left in the metal. This would require impractically large additions of reducing agent and electrical power with attendant carbon increases and too much slag volume. These problems are effectively avoided by the present process.

in some embodiments of the present process, it may be desired to oxidize a predetermined amount of chromium during decarburization to aid in maintaining the temperature of the molten stainless steel within a desired range.

A predetermined amount of chromium can be oxidized to produce heat in accordance with the present process by maintaining the oxygen volume percent at a value corresponding to a predetermined chromium content desired in the bath at the finish of the process. This final predetermined chromium will necessarily be less than the chromium content before the introduction of oxygen and inert gas, but, by the same token, the present process allows an artisan to control the amount of chromium oxidized in accordance with heat requirements desired during the present process.

For example, if it is desired to decarburize from 0.40 percent carbon to 0.0 r percent carbon while the chromium decreases from 20 to 16 percent and the temperature increases from 1600 C. to 1750 C., a blowing mixture containing about 40 percent oxygen should be injected to arrive at the stated final conditions. The proper blowing rate and blowing time to be used in conjunction with this example are a function of a specific furnace size, the desired temperature rise, furnace heat losses, and furnace heat capacity and can be calculated for a specific furnace by those skilled in the art,

Decarburization using inert gas in the final step of a programmed blow is illustrated by the following example.

'l'wo duplicate tests were run on a 25 pound scale in an induction furnace each test consisting of two decarburization steps: (1) decarburization to about 0.10% C by injecting an argon-oxygen mixture containing 48% by volume oxygen and (2)' final decarburization with argon injection only. The experimental results are compiled below:

These tests indicate that extremely low carbon levels may be achieved by the herein described methods without substantial oxidation of the chromium present in the melt.

This invention is further illustrated by the following examples which were conducted at varying oxygen concentrations in the injected inert gas-oxygen mixture. The

tests were carried out in an induction furnace. The experimental results are tabulated below:

Test No III IV V VI VII Vol.-percent Oz in AIS-O2 mixture 4. 6 17. 4 48. 48. 0 100 Final Temperature, O. 1, 675 1, 688 1,663 1, 659 1, 654 Initial Wt.-percent Cr... 19. 2 19. 6 18. 0 17. 1 19.1 Final nit-percent Or.. 19.1 18. 17. 1 16.3 16.0 Initial wt-percent O..- 0.025 0. 40 0.77 1.16 0.73 Final Wt.-percent O 0.005 0. 013 0.077 0. l2 0. 38 Percent O obtainable by prior art 02 lancing methods under similar conditions 0. 40 0. 35 0. 40 0. 39 0. 40

The above data illustrate the large degree of decarbm'ization obtainable by the process of this invention while substantially no chromium oxidation was taking place. Moreover, a check for test VII which employed 100% 0. was obtained. The final C content obtained was 0.38 wt.-percent whereas the prior art C-Cr-T relationships predicted 0.40 wt.-percent C.

This indicates that the experimental conditions during the above experiments were similar to those encountered during prior art oxygen lancing methods. The

Test N0 I II III IV Percent Or 17.1 16.3 19.1 18.5 Temp, 1, 936 1, 932 1, 942 1, 961 Percent 02 in inert gas-02 mixture..- 48 48 4. 6 17. 4 Percent; 0 actual 0.077 0.12 0. 005 0. 013 Percent 0 calculated 0. 12 0.12 0.001 0.016

The decarhurizing gas was injected into the melt and induction furnace was employed for melting the charge and maintaining the melt temperature.

We claim:

1. A process for removing carbon from molten steels containing about 3 to 30 percent chromium without substantial loss of chromium comprising adjusting the temperature of said molten steel bath to a range between about 1700 to 2300 degrees Kelvin, introducing into said molten steel containing from 3 to 30 percent chromium a decreasing ratio of volume percents of gaseous oxygen and at least one inert gas selected from the group consisting of argon, xenon, neon, and helium While maintaining said adjusted temperature within said range during said decarburization, said gaseous oxygen caused to react with said carbon to form a volatile carbon oxide to decarburize said molten steel, and said ratio decreasing as carbon is oxidized, the gaseous oxygen volume percent of the total volume of said gaseous oxygen and said selected inert gas in said varying ratio of volume percents of said gaseous oxygen and said selected inert gas being maintained substantially equal to the volume percent of gaseous ox gen resulting at any given carbon content during decarburization from the relationship Percent C where percent C equals the percent carbon content of said molten steel during said decarburization, where percent Cr is substantially constant and equal to the percent chromium content of said molten steel at the point in the process where inert gas is introduced into said molten steel, and T is the temperature in degrees Kelvin during said process and Within said adjusted range.

2. A process in accordance with claim 1 including tne additional step of introducing at least one inert gas into said molten steel after substantial decarburization is accomplished with said oxygendnert gas mixture.

3. A process for stepwise removal of carbon to a final predetermined amount in an otherwise finished molten steel containing a predetermined chromium content in the range from 3 to 30 percent chromium without substantial loss of said chromium during decarburization comprising adjusting the temperature of said molten steel to a range between 1700 to 2300 degrees Kelvin, introducing into said molten steel containing from 3 to 30 percent chromium, a stepwise decreasing ratio of volume percent of gaseous oxygen to at least one inert gas selected from the group consisting of argon, xenon, neon and helium, said gaseous oxygen caused to react with said carbon to form a volatile carbon oxide to decarburize said molten steel, and maintaining said adjusted temperature, without substantial decrease, within said range during decarburization and said ratio decreasing stepwise as carbon is oxidized, the gaseous oxygen volume percent of the total volume of said gaseous oxygen and said selected inert gas being decreased stepwise during decarburization to a value less than the volume percent of gaseous oxygen resulting from the relationship P cent O2 [Percent Cr H (13,s

Percent C an log T Where percent Cr approximately equals said predetermined chromium content in said finished steel, percent C equals any value above said final predetermined carbon content and equal to the carbon content desired at the end of the next subsequent decarburization step, and T equals the temperature of said steel during said process and is within said adjusted range.

4. A process for removing carbon from molten steel containing about 3 to 30 percent chromium wherein a predetermined amount of chromium is intentionally oxidized to provide heat to said molten stainless steel melt comprising introducing into said molten stainless steel containing from 3 to 30 percent chromium, a decreasing ratio of volume percents of gaseous oxygen to at least one inert gas selected from the group consisting of argon, xenon, neon and helium, said gaseous oxygen caused to react with said carbon to form a volatile carbon oxide to decarburize said molten stainless steel and said gaseous oxygen reacting with a predetermined quantity of said chromium to provide heat to said molten stainless steel the gaseous oxygen volume percent of the total volume of said gaseous oxygen and said selected inert gas in said varying ratio of volume percents of said gaseous oxygen and said selected inert gas being maintained substantially equal to the volume percent of gaseous oxygen resulting from the relationship 13,000 Pement O2=\/ [Percent Cr U 13,se0

Percent C an 1 0g T at a given carbon content during decarburization where percent C equals the percent carbon content of said molten stainless steel during said decarburization, percent Cr is substantially constant and equal to the percent chr0- mium content of said molten stainless steel at the point in the process Where inert gas is introduced into said References Cited in the file of this patent UNITED STATES PATENTS 1,034,785 reene Aug. 6, 1912 1,034,786 Greene Aug. 6, 1912 1,034,787 Greene Aug. 6, 1912 1,792,967 Clark Feb. 17, 1931 FOREIGN PATENTS 472,397 Great Britain Sept. 12, 1937 OTHER REFERENCES Journal of Iron and Steel Institute, vol. (1955), pp. 97-106, and 1l6l28. 

1. A PROCESS FOR REMOVING CARBON FROM MOLTEN STEELS CONTAINING ABOUT 3 TO 30 PERCENT CHRONIUM WITHOUT SUBSTANTIAL LOSS OF CHRONIUM COMPRISING ADJUSTING THE TEMPERATURE OF SAID MOLTEN STEEL BATH TO A RANGE BETWEEN ABOUT 1700 AND 2300 DEGREES KELVIN, INTRODUCING INTO SAID MOLTEN STEEL CONTAINING FROM 3 TO 30 PERCENT CHROMIUM A DECREASING RATIO OF VOLUME PERCENTS OF GASEOUS OXYGEN AND AT LEAST ONE INERT GAS SELECTED FROM THE GROUP CONSISTING OF ARGON, XENON, NEON, AND HELIUM WHILE MAINTAINING SAID ADJUSTED TEMPERATURE WITHIN SAID RANGE DURING SAID DECARBURIZATION, SAID GASEOUS OXYGEN CAUSED TO REACT WITH SAID CARBON TO FORM A VOLATILE CARBON OXIDE TO DECARBURIZE SAID MOLTEN STEEL AND SAID RATIO DECREASING AS CARBON IS OXIDIZED, THE GASEOUS OXYGEN AND SAID OF THE TOTAL VOLUME OF SAID GASEOUS OXYGEN AND SAID SELECTED INERT GAS IN SAID VARYING RATIO OF VOLUME PERCENTS OF SAID GASEOUS OXYGEN AND SAID SELECTED INERT GAS BEING MAINTAINED SUBSTANTIALLY EQUAL TO THE VOLUME PERCENT OF GASEOUS OXYGEN RESULTING AT ANY GIVEN CARBON CONTENT DURING DECARBURIZATION FROM THE RELATIONSHIP 